I find this hard to believe considering how guys like Cro Cop needed to take a break after getting hit by their foes. And these guys were wearing cups. And they were pros who did what they did for a living.
I've seen dudes in rel life as well who get hit in the nuts.
When fighting competitively and fighters get hit in the nuts they will take the maximum amount of time the ref allows to get back, not because it hurts so much but it gives them a quick breather and more importantly unsettles their opponent ( not foes, they are actually likely to be mates outside the cage) what you see in competitive fights isn't always what is actually happening.